1. FIELD OF THE INVENTION
This invention relates to vinyl polymer-type electrostatic spray coating powder paint compositions and processes for preparing the same.
2. DESCRIPTION OF THE PRIOR ART
Heretofore, there have been conventionally used solution-type paints in which the paint components are dissolved in an organic solvent or emulsion-type paints in which the paint components are dispersed in an aqueous medium.
However, the former may possibly cause environmental pollution due to the evaporation of the organic solvent from the paint following application thereof. The latter has drawbacks because the film derived therefrom is lacking in desired physical and chemical properties. Accordingly, powder paints which do not use liquid carriers or vehicles have recently become available. Coating processes using these powder compositions have been developed. Domestic utensiles, steel furniture, general purpose piping, automobile parts, etc. are painted with such powder paints. There are various modes for painting with such dry paint compositions. Recently the electrostatic dry spray painting has become important. In this process the paint powder suspended in air is charged with a high voltage while it is flowing through the air to cause some electrostatically to adhere onto the article being painted. The article is heated to melt the powder and thereby form a paint film.
Vinyl powder paints conventionally used as electrostatic dry spray coating powder paints are produced by the following methods:
1. A dissolution-deposition process in which, after the vinyl resin is dissolved in a solvent and additives such as pigments, plasticizers, stabilizers, etc. are added thereto, a non-solvent is added thereto with vigorous agitation of the mixture to precipitate the powder paint composition.
2. A fusion-pulverization method in which the resin is heated until it is molten, pigment and coating film forming elements are added thereto and mixed therein, and after the mixture is solidified, the solid mixture is pulverized.
3. A dry blending method in which the resin, pigment and coating film forming elements are mixed by a high speed rotary mixer.
However, the first method (1) must employ a solvent, and it is further necessary to carry out the steps of dissolving the resin, precipitating the powder paint particles and recovering the solvent. In the second method (2) the resulting powder particles thereby obtained are broken pieces so that it is difficult to form therefrom a coating film having a flat and smooth surface. In the third method (3) there can be obtained only resin particles which are externally coated with the pigment particles.
Recently there have been proposed the following various methods in addition to the aforementioned methods.
In one method, there is disclosed a method of producing a powder paint in which a vinyl chloride monomer, or a vinyl chloride monomer and comonomers such as vinyl acetate, vinyl ether, etc., is suspension-polymerized in the presence of carbon black and using an oil-soluble polymerization initiator (organic peroxide) such as diisopropylperoxydicarbonate, azobisisobutyronitrile (Japanese Open-Laid Patent Publication No. 29475/72). However, according to this method, the pigment is not sufficiently dispersed in the powder particles, and its polymerization yield is remarkably poor. In addition, the size of the obtained powder particles is not consistent and their thermal stability is poor, and further, blocking thereof occurs during storage.
Japanese Open-Laid Patent Publication No. 976/73 discloses a method of obtaining a unplasticized polyvinyl chloride composition adapted for machining in which the polymerization is effected with the addition of agents such as stabilizer, lubricating agent, pigment, polymer denaturing agent, generally contained in unplasticized polyvinyl chloride compositions, before or during the polymerizing step and then adding a polymerization stopping agent to stop the polymerization. This method has several disadvantages, namely, that the pigment is not uniformly distributed in the interior of the polymer particles, the diameters of the particles are irregular, and the diameters of the particles are not controlled, and the polyvinyl chloride thus obtained is poor in thermal stability.
Japanese Open-Laid Patent Publication No. 10181/73 discloses a method of obtaining dispersible coating compositions which comprises mixing a resin containing carboxyl groups as a starting material with pigment, neutralizing the carboxyl groups in the mixture, emulsifying the mixture in water, adding a polymerizable monomer into the mixture and then effecting polymerization. Japanese Open-Laid Patent Publication No. 10182/73 discloses a method of obtaining a coating composition dispersed in water which comprises dispersing a pigment in a water-insoluble resin, then emulsifying the dispersion in water by using an emulsifying agent or dispersing agent, and adding a polymerizable monomer thereto and then polymerizing it. However, these methods are processes for producing coating compositions to be coated in the form of a dispersion in water. The diameters of the polymer particles of polymer are about 1 micron, and these are almost the same as the diameters of the particles of the polymer obtained by the conventional emulsion-polymerizing method which are unsatisfactory as an electrostatic powder paint.